Field of invention The invention relates to a dome shaped roof for a cryogenic tank having a very large diameter, commonly larger than 50 metre, and to a method for constructing such roof using formwork which structurally supports the roofing during the construction phase and which may be removed subsequent to construction.

Short description of the prior art Reguere, R. F. , Rivas, J. L., Legatos, N. A. and Marchaj, J. describes in the article entitled"An 80.000 m3 Double Prestressed Concrete Wall LNG Tank: Its Design, Construction and Commissioning", an LNG tank having an inner primary tank provided a steel casing arranged externally of the inner primary tank. The tank according to Reguera et al is further provided with a bottom insu- lation consisting of several layers of PVC plates placed on a vapour barrier, the PVC plates being covered by 5mm thick 9% Ni lap-welded bottom plates. In order to protect the PVC plates from rain during the construction phase,

the dome shaped or cupola shaped roof was constructed prior to constructing the insulated bottom of the primary tank. A steel dome supporting structure consisting of 88 ribs resting on a ring beam and supported centrally by means of a temporary arranged ring structure was erected and covered by steel plates or panels. The steel dome supporting structure was erected on the bottom of the tank in parallel with concreting the upper part of the inner wall. The steel dome was lifted to its permanent position subsequent to completion of construction of the concrete wall forming the outer, secondary tank. The cupola or doom constructed inside of the primary tank according to Reguera has, however, a smaller diameter than the ring beam on to which it is to be supported, on top of the cylindrical secondary tank wall. The solution of this problem requires that a lower, ring formed part of the dome is to be constructed on top of the ring beam, the lower portion of which having an inner diameter which corresponds to the diameter of the steel doom part to be erected within the primary tank and lifted up to its permanent position. Assembling the two parts require additional steel. The steel dome is then covered by reinforced concrete, poured in-situ. The Reguera solution does not provide any re-use of the steel dome structure for use in a next, subsequent construction of a dome.

Japanese patent application 4-302799 discloses a method for constructing a dome shaped roof on a low temperature tank of the underground type. The Japanese patent discloses an inner, cylindrical primary tank wall which is not capable of supporting the dome, and an outer, cylindrical secondary tank wall supporting the tank wall.

A circumferentially arranged knuckle plate, forming the lower circumferential part of a dome is erected around the top of the outer secondary tank. The circumferentially arranged knuckle plate has an inner diameter which

corresponds to the diameter the primary tank, as for the Reguera tank. A centre portion forming a fundamental roof block for the dome is constructed. The fundamental roof block serves as a bridge across the inner diameter of the knuckle plate. The fundamental roof block is formed by erecting two mutual opposite, radial roof elements on opposite side of a circular mid-section and erecting the fundamental roof block unit to span a meridional arc across the knuckle plate. Then corresponding radial roof elements are then erected between the knuckle plate and the basic roof block until the entire dome is constructed.

The dome is then covered with plates resting on the radial roof elements. A drawback with the JP 4302799 solution is the need for the circumferential knuckle plate, covering the space between the outer (secondary) tank and the inner (primary) tank. A further disadvantage with this prior art solution is the necessity of constructing the fundamental roof block unit inside the primary tank or outside the primary tank and then lifting it so as to span across the circumferential structural section, requiring a large lifting capacity at the construction site. A central"work platform"for use during assembly of the fundamental roof block for the outer secondary tank is known from the Japanese patent specification. In order to form a suspended insulating roof for the primary tank, such central work platform will also be used to assemble parts while the fundamental roof block rests on the ground.

An alternative method for constructing a dome is to construct a circumferential plate for the dome as described above, and then construct a dome calotte on the bottom of the primary tank using standard scaffolding, covering the entire primary tank floor, and the pump the dome calotte upwards until it may be assembled with the circumferential plate. Such dome has again the disad- vantage that it requires separate construction of a dome

calotte and a circumferential plate. It is desirable to build the dome substantially as one monolithic element, avoiding formation of one circumferential knuckle plate, as the construction of such plate is time consuming.

JP 5280222 describes a balloon like membrane extending across the top of a vertical cylindrical tank wall. The membrane is pressurized so as to give the membrane the required dome shape. A layer of concrete is then poured on top of the membrane. The method according to JP 5280222 avoids the use of a conventional form work.

In order to avoid flattening of the central part of the dome formed by the balloon, the circumferential parts of the poured concrete are thicker than the layer of concrete poured at the central part of the dome. When using air pressure the small tolerances required for the shape of a dome shaped cupola will not be met. Wind from any direction during the construction phase may change the shape of the dome causing deviation from the required spherical shape.

Japanese patent application JP 2001-81866 describes a method for constructing a cupola shaped or a dome shaped roof for a stadium. The method comprises construction of a ring shaped body where pre-cast concrete elements are arranged sequentially around the circumferential direction of the upper end of a lower structure to form a ring-like connecting body. Tensile force is introduced both in the circumferential direction and diametrical direction. Only a circumferential part of the dome is constructed by using the method described above. A central membrane part of the roof is constructed on top of several working platforms or support towers and thereafter jacked up and assembled with the circumferentially constructed concrete shell. As for the previous described prior art, the disadvantages of constructing the dome in to parts, also having different material properties, are considered detrimental.

Summary of the Invention A solution solving the problems described above is to construct a dome shaped, insulated roof structure for a vertical, cylindrical tank having a large diameter, where the dome shaped roof structure rests on a vertical, cylindrical wall and where the dome shaped roof structure comprises a trusswork comprising a plurality of radially arranged trusses made of wood, each truss having an upper chord and a lower chord, the former having a total shape which corresponds to the intended shape of the dome, while latter preferably being horizontal; the trusses, which preferably are prefabricated, are interconnected by means of beams extending in lateral direction with respect to the longitudinal direction of the trusses; a first tight structure formed by metal cover made of thin metal plates, supported by the upper chord of the trusses and on the interconnecting beams; and a concrete layer, preferably pre-stressed or post- tensioned, poured on top of said first tight cover.

An alternative solution for solving said problems is to construct a vertical, cylindrical tank having large diameter, for storage of cryogenic condensed gas, such as propane, methane, nitrogen, etc. The storage tank comprises an inner primary tank and an outer secondary tank supporting a dome shaped or cupola shaped roof resting on an upper, secondary tank ring beam arranged on top of a vertical, cylindrical secondary tank wall of the secondary tank, wherein the dome shaped roof comprises a first, lower, dome shaped layer of formwork elements arranged on a dome shaped temporary trusswork of meridionally arranged, upwardly convex arced girders, wherein each consecutive

pair of girders spans over a circle sector of the dome shaped roof ; each girder is supported at its meridional outer end by means of the secondary tank ring beam; each girder is supported near its meridional inner end by means of a temporary support tower; a second layer of concrete is cast in-situ on top of the layer of formwork elements, the second layer being more than self-supported when the concrete has cured; a third layer of concrete cast in-situ on top of the second layer, the third layer at least partly being supported by the second layer in its cured condition and partly by the first layer, the third layer also being partly supported during casting and curing by the dome shaped formwork supported by the temporary support tower.

Further, the invention relates to a temporary, structural support for construction of the dome shaped roof, having large diameter, on a horizontal ring beam.

The temporary support comprises at least a temporary support tower for formwork, resting on a supporting foundation. The tower is arranged centrally inside the tank and is intended to support a dome shaped, temporary formwork of radially arranged girders, the upper chord of which having an upwardly convex arc and where each consecutive pair of beams spanning across a circle sector of the secondary ring beam. Each girder is supported at its outer radial end by the ring beam.

According to an embodiment the girders are removed when the concrete layer has cured and is self-supported.

According to a second embodiment the supporting girders remain in an installed position below the self- supported, cured concrete roof, suspended from said roof.

According to this embodiment the supporting girders are structurally supporting the concrete roof only during the concreting phase until the concrete roof has become self-

supported, whereupon the supporting girders only serve as support for the insulation.

The invention comprises also a corresponding method for constructing a dome shaped roof for a tank having very large diameter, the tank being intended for storage of cryogenic condensed gases of propane, methane, nitrogen, etc. The embodiment is characterized in that trusses are arranged in a radial orientation, each truss being supported by a temporary support tower centrally arranged within the cylindrical tank, while the opposite ends of the trusses are supported by the cylindrical tank wall.

Transverse beams are arranged between the radial girders, interconnecting two adjacent girders, the upper surface of the upper chord of the girders and the corresponding surface of the transverse beams being given a shape which correspond to the required dome shape. A tight membrane formed of thin plates is arranged on top of the girders and the transverse beams. A reinforced layer of concrete is cast in-situ on top of the thin plated layer, whereby the thin plated membrane functions as formwork for the concrete layer. The temporarily arranged support tower is removed when the concrete has cured and has become self- supported.

According to a further embodiment one or more temporary support towers are erected within a vertical, circular, cylindrical primary tank side wall, whereupon a plurality of radially arranged girders, forming a dome shaped trusswork extending between an upper end of the temporary tower and above the side wall, and also supported at their radially outer ends, are installed between the temporary tower and the top of the ring beam on top of the tank wall. Each consecutive pair of girders spans across a sector of the ring beam. A first layer of formwork elements are then installed and fixed to the girders and beams, covering the entire dome except for a

small opening at the top of the dome shaped trusswork, whereupon a second layer of concrete are concreted on the formwork elements. The second layer of concrete is cured and becomes a more than self-supported concrete layer fixed to the first layer of formwork elements. A third layer of concrete is then concreted on top of the second layer of concrete, the third layer being supported at least partly by the cured second layer.

One of the advantages according to the invention is the possibility of constructing a monolithic dome on the outer secondary tank, wherein the dome has a diameter larger than the diameter of the inner primary tank, thereby avoiding that the lower part of the dome is constructed separately from the central main part of the dome. Hence, both weight and required construction time is reduced. Further, such method provides a more homogenous, monolithically reinforced concrete dome without joints. It is thus feasible to construct a cryogenic tank dome having larger diameter than previously possible. A further advantage according to the invention is the feasibility of dismantling and removal of at least the support tower, and possibly also the support girders, after the reinforced concrete has been cured. The structural part of the formwork, represented by the girders and the support tower, may be re-utilized for constructing a dome at another site comprising several storage tanks. The structural steel support elements of the formwork may also easily be transported to remote sites for construction of a storage tank dome.

Brief Description of the Drawings Figure 1 shows a vertical section through a cryogenic tank according to the invention, having an inner primary tank comprising a cylindrical, reinforced concrete wall, and an outer secondary tank also having a cylindrical,

reinforced concrete wall. The section illustrates a dome or cupola shaped formwork according to the invention, the structural part of which comprising steel braced girders supported centrally by a supporting column which may be dismantled together with the steel girders and removed subsequent to casting of the reinforced concrete dome. The structural parts of the form work are removed through a relative small opening in the concrete dome.

Figure 2 illustrates a section through a part of the dome shaped structural elements of the formwork along one steel girder, and through a first prefabricated concrete shuttering plate used as formwork for a second layer of reinforced concrete cast in-situ, a gastight membrane and a third layer of reinforced concrete, also cast in-situ.

Figure 3 is an isometric sketch and a vertical section of a cryogenic tank according to the invention at a late stage in the construction process. A self- supporting concrete dome has been concreted on top of the secondary tank wall, and an insulated roof is suspended from the dome.

Figure 4 is a section through the upper part of the primary tank wall and the secondary wall, supporting the steel girders in the girders forming the dome shaped formwork, shown in an isometric perspective.

Figure 5 is a top view of the dome shaped structural trusswork showing a limited number of formwork plates.

Figure 6 is an isometric view of the formwork system seen separated from the cylindrical tank walls within which it is erected during the construction period.

All parts shown on the Figures will be removed from the tank at the end of the construction period, subsequent to casting of a reinforced concrete dome.

Figure 7 shows in perspective a vertical section through a storage tank according to the invention with the

dome shaped roof structure in place.

Figure 8 shows in perspective view the storage tank according to the invention prior to concreting the dome shaped roof structure.

Figure 9 shows in perspective a vertical section of an upper part of the storage tank and the trusses, resting on the tank wall.

Description of the preferred Embodiments of the Invention Figure 1 shows a section through a cryogenic tank dome constructed according to the present invention. The tank is a vertical, cylindrical tank having a large diameter, for storage of cryogenic condensed gases, such as propane, methane, nitrogen or the like. The preferred embodiment comprises an inner primary tank for storage of the condensed gas. The primary tank has a cylindrical, reinforced concrete wall. The tank comprises also an outer cylindrical, reinforced secondary tank formed by a cylindrical, reinforced concrete wall. The section shown in Figure 1 illustrates a dome shaped or cupola shaped layer 5 of formwork made of formwork plate elements 6.

According to the invention the formwork layer 5 is supported by a structural part comprising steel girders 10, supported according to the preferred embodiment mainly in the middle by means of a centrally arranged supporting tower 12. The supporting tower may be dismantled together with the steel girders 10 and removed subsequent to casting and curing of the reinforced dome. The structural steel elements in the formwork will be removed through a relative small opening 51 at the top of the concrete dome.

The dome is according to the invention constructed in the following way: The tank comprises an inner primary tank 1 and an outer secondary tank 2, supporting a dome shaped or cupola shaped roof 3. The dome shaped roof 3 rests on a horizontal, upper secondary tank ring-shaped

beam 42 constructed on a vertical, cylindrical secondary tank wall 4 on the secondary tank 2. The dome shaped roof 3 is made of a first, lower dome shaped layer 5 formed by formwork elements 6 placed on dome shaped, temporary supporting trusses 8. The trusses 8 are made of meri- dionally arranged beams 10 being upwardly, convexly curved. Each consecutive pair of beams 10 spans a circle sector of the dome shaped roof 3. Each girder 10 is supported at a meridional outer end by the ring beam 42 on the secondary tank. Each girder 10 is further supported near its meridional inner end by a temporary supporting tower 12, which according to the preferred embodiment is erected on the bottom slab of the primary tank 1.

The layer of formwork elements 6 is then installed and optionally fastened on the girders 10. A second layer 14 of concrete is cast in-situ on top of the layer 5 of formwork elements 6. The second concrete layer 14 becomes more or less self-supported when the concrete has cured.

In cured condition the layer 14 is preferably fixed to the formwork elements 6. The second, cured concrete layer is designed to be more or less self-supported so that it may support a consecutive, preferably thicker layer 18 of concrete to be cast in-situ.

The third concrete layer 18 is cast in-situ on top of the second concrete layer 14. The third concrete layer 18 is supported at least partly by the second, cured concrete layer 14 and the first layer 5. According to a preferred embodiment the third layer 18 may be partly supported, during casting and curing by the dome shaped trusses 8, supported by the temporarily arranged support tower 12.

The temporary support tower 12 may be lowered slightly in order to mobilize the load bearing capacity of the second concrete layer 14, causing the layer to yield a few millimetres or centimetres during casting of the third layer 18.

According to a preferred embodiment of the invention the dome comprises a gastight membrane 16 arranged between the second concrete layer 14 and the third concrete layer 18, ref. Figure 2. The gastight membrane 16 is applied to the second concrete layer 14 prior to casting of the third concrete layer 18. The gastight membrane may comprise asphalt.

Figure 2 illustrate that the tank according to a preferred embodiment of the invention, may comprise pre- fabricated formwork elements 6 in the lower, dome shaped layer 5, and may be provided with spherical surface and made of reinforced concrete. According to a simplified alternative the formwork elements 6 may be plane and be so small compared with the spherical radius of the dome that the deviation from the ideal spherical form is acceptable.

According to a preferred embodiment of the invention the temporary support tower 12 supports a temporary, central, inner ring beam 122, supporting the meridional inner ends of the beams 10.

Figure 2 illustrates further the reinforcement of the concrete layers. The second concrete layer 14 may be rein- forced by meridional reinforcement bars 142 and also by circumferentially arranged reinforcement bars 144. Corre- spondingly the third concrete layer may be reinforced by meridional reinforcement bars 182 and circumferentially arranged reinforcement bars 184.

According to a preferred embodiment of the invention the horizontal, upper secondary tank wall ring beam 42 and the outer, vertical, cylindrical secondary tank wall of the outer secondary tank 2 may be made of reinforced concrete. The inner primary tank 1 comprises a cylindrical primary tank wall 41 also made of reinforced concrete. The two cylindrical side walls 4,41 may be constructed in a parallel slipforming process. The inner primary tank 1 may comprise a horizontal primary bottom plate made of

reinforced concrete. Alternatively, the bottom plate may comprise a steel plated wooden floor 15 supported by a grid of wooden beams placed on the ground, the beams resting on a secondary bottom plate or foundation 22 formed by concrete, preferably made as a monolithic structure with a foundation ring beam 23 for the secondary tank, supporting the outer cylindrical side wall 4 of the secondary tank 2.

The primary tank 1 of the tank according to the invention comprises a primary bottom plate 15 and primary tank side walls 41, separated from the secondary tank 2, thereby providing a space between the cylindrical walls and a distance between the two bottom plates which at least is filled with insulating layers 44,44B.

The preferred embodiment of the tank according to the invention is provided with a number of structural prestressing suspension tendons 52 arranged in such manner that they are extending vertically down, e. g. from pre- stressing anchors 53 placed in the third concrete layer 18 of the dome shaped roof 3. Such anchors may be of the active or the passive type. An active anchor is shown in Figure 2. The suspension tendons may be used to suspend a primary tank roof 60. The roof 60 comprises horizontal roof truss 62 for the primary tank which partly rest on the inner, upper primary tank ring beam 21 on top of the cylindrical primary tank side wall 41. On its top, the truss 62 according to the preferred embodiment is covered by corrugated plates 64 of aluminium, supporting a layer of insulation 66 for the roof in the primary tank.

According to the preferred embodiment of the invention a number of anchors 53 for tendons are arranged in the third concrete layer 18 in order to support the insulating roof 60 in the primary tank. Each anchor 53 is intended to lock the tendons in a position where the tendons are hanging vertically down from and through the

dome shaped roof 3. Alternatively, the anchors 53 may be replaced by one or more large anchor rings 53b made of steel and embedded in the third concrete layer 18, concentrically around the upper opening 51, the anchor rings being used to support the vertically, downwards extending suspension tendons 52.

The tank according to the preferred embodiment of the invention comprises a central opening 51 through the first dome shaped layer 5, the second concrete layer 14 and the third concrete layer 18. The purpose of the opening 51 is to allow removal of the temporary structural parts of the formwork, such as the girders 10 and the temporary support tower 12 subsequent to casting of the concrete layers 14, 18. Required pipelines, power supply cables, maintenance tools and measuring instruments may also easily be intro- duced through the central opening 51, thereby limiting the number of lead-ins through the dome which may weaken the structural integrity of the dome. A possible lead-in of pipelines etc. arranged through the inner primary and secondary cylindrical wall 41,4 may also reduce the integrity and the strength of the tank walls.

The Formwork A further aspect with the invention is related to a formwork having a temporary structural support for casting of the dome shaped or cupola shaped concrete tank roof 3 with large diameter. The structural part of the formwork is illustrated in the Figures 5 and 6. The dome is prefer- ably supported by the horizontal ring beam 42 arranged at top end of the outer secondary tank wall 4, as illustrated in Figure 4. Alternatively, the dome may'be constructed directly on top of the secondary tank wall 4, without using a ring beam. The formwork remains supported by a structural support system comprising at least one tempo- rary support tower 12 erected on a supporting foundation,

such as the bottom plate 15. The support tower 12 is arranged internally of the ring beam 12. The tower 12 has a central, temporary inner ring beam 122 designed to support a dome shaped temporary trusswork 8 made of meridionally arranged convexly curved beams 10. Each consecutive pair of beams 10 spans across a circular sector of the secondary ring beam 42 and the temporary, inner ring beam 122. Each of the beams 10 is at its meridional outer end (meridional in relation to the top of the dome structure) supported by the largest ring beam 42.

The first, lower, dome shaped layer 5 of formwork elements 6 are placed on top of and anchored to the beams 10 in the dome shaped, temporary trusses 8, as indicated in Figure 5 by means of a circumferential sector ring consisting of five consecutive formwork elements 6 followed by three adjacent formwork elements 6 arranged inside and above the outer, lower sector ring. In this manner the entire dome trusswork 8 is covered by formwork elements prior to in- situ casting of the second concrete layer 14.

The second concrete layer 14 is cast in-situ on top of the layer 5 of formwork elements 6. According to a preferred embodiment of the invention the second concrete layer 14 is more than self-supported when cured and is fixed to the formwork elements 6. The second concrete layer 14 serves as formwork for the third, upper layer 18 in-situ cast on top of the second concrete layer 14. The third layer 18 is at least partly supported by the cured second layer 14 and the first layer 5. The third layer 18 may, however, also partly (indirectly) be supported during casting and curing by the temporary, dome shaped trusswork 8, supported by the temporary support tower 12. As explained above lowering of the support tower 12 by a predetermined distance during casting of the first layer 5, may bring about the load bearing capacity of the second concrete layer 14 which then is allowed to yield a few

millimetre or centimetre during casting of the third layer 18.

A part of a beam 18 is illustrated in the lower part in Figure 1. In a preferred embodiment of the invention each girder 20 in the temporary, structural trusswork 8 supporting the first concrete layer 5, comprises two upper, curved girder elements llla, lllb, arranged to extend between the ring beam 42 and the support tower 12.

The two girder elements lla, llb extend in its operative position parallel and are evenly distanced apart. The operative position of the girders 10 is illustrated in Figure 4. Each of the girder elements llla, lllb of the girder 10 defines an upper convex curve. They are interlinked by means of a number of smaller, angular struts 113. The upper girder elements llla, lllb are also separately connected with a lower girder element 112 by means of a number of angular struts 115, as illustrated in Figures 1,2, 4 and 4b. In order to obtain a required spherical shape for the formwork, the upper girder elements llla, lllb are formed as a circular curve, upwards convex in their erected position. A cross-section of the girder shown in Figure 4b shows two upper girder elements llla, lllb forming the corner in a base line of a isosceles triangle where the lower girder element 112 defines the third corner of the triangle.

According to the simples and most preferred embodiment of the invention the temporary support tower 12 is erected in the middle of the tank, i. e. centrally with respect to the upper ring beam 42 on the secondary tank and centrally on the base plate 15 of the primary tank. In order to support the girders 10 at a meridional inner end of each of the girders 10 for tanks having very large diameters, two, three or more support towers may be used, thus reducing the free girder span. Such arrangement will, however, increase the required period of construction and

disassembling the formwork. Such solution will also require a more complicated dome shaped formwork 8.

It is not practical to interconnect all the girders 10 with each other at the middle of the apex of the dome structure. The formwork supported by the girders 10 according to the invention is thus supported at its meridional inner end by a central, temporary inner ring beam 122 having a small diameter compared to the diameter of the upper secondary ring beam 42. The temporary, inner ring beam 122 is designed to support the inner ends of the girders 10 and is supported by inclined struts 122b, arranged near the top of the temporary support tower 12, see Figure 1 showing a vertical section and Figure 6 showing an isometric view.

The preferred embodiment of the formwork elements 6 when assembled, constitute the formwork in the form of a dome shaped layer 5. The formwork may be formed of prefabricated units and may provide a spherical surface.

This may easily be achieved by constructing a formwork with a partly spherical surface, e. g. at the construction site, giving the formwork a length and a width which are larger than the largest required element 6 for casting each element 6. The shape of consecutive meridional types of formwork elements 6 depends on several parameters such as the radius of the dome from near the middle of the tank, the meridional distance from the top of the tank and the arc of the ring beam to be spanned. An element 6 may span across more than two girders 10 in the region close to the top of the dome shaped trusswork 8 as illustrated in Figure 5. The formwork elements 6 are preferably made of prefabricated concrete. Alternatively, the formwork elements 6 may be made of steel.

In order to lower the supporting girders 10 a few millimetre or centimetre from the concrete layer 5, the temporary support tower 12 may be provided with jacks 102

enabling lowering or elevation of the tower 12. The jacks are indicated in Figure 6 and may be of any suitable type.

The jacks may comprise hydraulic pistons or any mechanical lifting means.

Construction of the Dome A preferred method for constructing a preferred embodiment for construction of the dome shaped roof 3 for a tank having very large diameter, for storage of condensate gas such as propane, methane, nitrogen etc, may comprise of the following steps: A temporary tower 12 is erected inside a vertical, circular cylindrical primary tank side wall 4, the tower 12 extending above said wall 4. Two or more towers may be used if the tank diameter is too large for one central tower, but such solution is not so advantageous due to increased construction time, price and complexity.

Preferably, but not absolutely necessary, the side wall 4 supports an upper, secondary tank ring beam 42 having a circumferential reinforcement in order cater for lateral forces imposed by the dome and the previously described steel beams during the construction period; and also forces imposed by the dome subsequent to the curing phase.

A cylindrical, temporary inner ring beam 122 is preferably supported by struts or beams 122b in the vicinity of the upper end of the temporary tower 12.

Meridionally arranged beams 10, forming a dome shaped trusswork 8, are then lifted to its operational position, extending between the inner ring beam 122 on the temporary tower 12. The beams rest at their meridional outer ends on the secondary tank ring beam 42, each consecutive pair of beams 10 spanning over a sector of the secondary tank ring beam 42. In the absence of a secondary tank ring beam on top of the cylindrical secondary wall 4, the top of said wall 4 supports the beams 10 directly.

The first formwork layer 5 consisting of formwork elements 6 is then placed on the beams 10 and fixed. The formwork elements 6 cover mainly the entire area apart from a relatively small part of the dome shaped trusswork 8, leaving a central opening 51 at the top of the dome shaped trusswork 8. The purpose of this opening 51 will be described above.

Subsequent to the installation of the formwork layer, a second concrete layer 14, illustrated in Figure 2, is cast on top of the formwork layer 5. This is the first concrete layer to be cast in-situ, but constitutes the second concrete layer 14. The second concrete layer 14 is cured. According to a preferred embodiment the second concrete layer 14 is more than self-supported, either alone or together with the first formwork layer 5 of formwork elements 6.

Subsequent to concreting the second concrete layer, a third concrete layer 14 is cast in-situ on top of the second concrete layer 14. According to a preferred embodi- ment of the invention said third layer 18 is supported at least partly by the cured second layer 14, as further described below.

According to a preferred embodiment of the invention the tower 12 is lowered by means of the jacks 102, thereby lowering the trusswork 8 of girders 10 a predetermined distance below the formwork layer 5. Release of the supporting force below the formwork elements 6 will mobilize the load bearing capacity of the cured second layer 14. Such load bearing capacity must be dimensioned to be at least sufficient to carry the weight of the third concrete layer 18 to be cast in-situ; and the weight of the second layer 14 itself and also the weight of the first formwork layer 5.

Since the three concrete layers 5,14, 18 will yield slightly due to the increased load of the third layer 18

the trusswork 8 of girders 10 will then come in contact with the first formwork layer 5. The trusswork 8 will then partly support the concrete layers when the first formwork layer 5 and cured second layer slightly yield as a consequence of casting of the third concrete layer 18.

When the third concrete layer has been cured, all girders 10 in the dome shaped trusswork 18 are dismantled and all girders 10 are removed through the central opening 51. The temporary support tower 12 may also be dismantled and lifted out through the opening 51.

Figure 7 shows in perspective a vertical view of a completed tank 200 according to the invention, showing the phase where a concrete roof forming the roof structure 205 of the tank 200, is concreted, but not yet self-supported.

The tank 200 comprises an outer secondary tank 201 and an internal concentric storage tank having vertical wall (s) 202. As shown in the Figure radially arranged trusses 203 extend between a centrally arranged, supporting tower 204 and the top of the tank walls 201,202. The tower 204 is temporarily arranged inside the tank 202, supporting the trusses 203 and the dome shaped roof structure 205. As indicated in Figure 7, the trusses have a radial orientation.

The trusses 203 comprise an upper chord which is given an upper surface adapted to the required shape of the dome. The trusses 203 are further interconnected, at least between the respective top chords. Also the top surfaces of such interconnecting beams are given a shape adapted to the dome shape of the roof (not shown). The purpose of the interconnecting beams is to interconnect the trusses and to form a safe support for thin steel plates to be placed on top and between the trusses and beams and welded together, thereby forming a fluid tight membrane which additionally also serves as formwork for a reinforced concrete layer forming the structural part of

the dome. Further, the trusses are provided with lower chords being substantial horizontal.

Figure 8 shows in perspective a horizontal view of a storage tank 200 prior to mounting the plates and rein- forcing and concreting the dome shaped roof structure 205.

Figure 8 shows the orientation of the radially arranged trusses 203. The interconnecting girders are not shown.

Figure 9 shows in perspective a vertical section of parts of the upper part of the storage tank 202 and the trusses 203. As shown in Figure 9 each truss may be provided with one or more intermediate chords and inter- mediate diagonal braces 207 and/or vertical columns or struts 208.

Insulating material (not shown) may be placed between at least the lower chords of the trusses. Alternatively the insulation material may be suspended from the trusses, e. g. from the lower chords 208 e. g. by means of tendons or the like, as shown in Figure 3.

When constructing such embodiment both the secondary tank 201 and the inner tank 202 are firstly constructed up to the level of support for the dome shaped roof structure 205. The space between the inner and the secondary tank is insulated in parallel. The inner, temporary tower 204 is then erected whereupon the trusses are mounted with one end resting on the tower 204 and the other end resting on top of the completed tank wall (s). The interconnecting beams, interconnecting each truss 205 with the next truss, are then mounted. Prior to the installation of the trusses, each upper chord is given an upper surface adapted to the intended dome shape of the roof structure.

Correspondingly, the upper surface of each interconnecting beam is given a shape corresponding to the intended shape of the roof structure.

The plates forming the fluid tight membrane in the roof structure and additionally serving as formwork during

concreting are then installed and inter-welded. Preferably the plates may also be given as shape corresponding to the intended shape of the roof structure.

Reinforcement is placed on top of the welded plates whereupon the roof structure is concreted and optionally prestressed or post-tensioned. When the concrete is cured and the roof structure is self-supported, the temporary tower 204 is dismantled, e. g. as described above, while the trusses 203 remain suspended from the self-supported roof structure. In this operational phase the trusses have no longer any structural function, supporting the concrete roof structure. The trusses are, however, used for support and/or suspension of the insulation.

As for the embodiments shown in Figures 1-6, an opening is left in the roof structure for removal of the tower 204 upon completion of the concreting process of the concrete roof. The opening is then closed by casting the remaining part.

By dome shape is meant a roof shape which may be formed by plane plates forming an angle between each other, or formed by single curved plates or double curved plates.